Sealants useful in aerospace and other applications are often pre-mixed frozen compositions (PMF) or two-part systems. In two-part systems, a first component contains a main polymer, such as a polysulfide polymer and/or a polythioether polymer, together with a number of additional materials. The first component does not contain a curing agent, which is included in the second component. The two components are manufactured and packaged separately, and mixed together immediately before use.
Unlike two-component systems, which require mixing the curing paste and the base before use, PMFs may be cured by external factors, such as temperature. For this reason, PMFs must be frozen at, for example, −40° F. to −80° F. in order to suppress or slow the curing reaction. When the PMFs are later brought to room temperature, the curing rate increases significantly. PMFs offer the convenience of being ready for use without mixing and therefore can be more cost- and time-effective than certain two-part systems. However, existing PMFs have limited pot lives, must be stored at very low temperatures such as −40° F. to −80° F., and require mixing of the base component and the activator followed by immediate freezing to slow the curing reaction. The requirement to store PMFs at low temperature prior to use can be inconvenient and/or expensive.
As a result, it is desirable to provide one-part aerospace sealant compositions that exhibit long pot life at ambient temperature and, when applied to a substrate and exposed to moisture (such as atmospheric moisture), cure to form a cured sealant having acceptable properties, including good shear strength. Moisture-curable sealant compositions in which the sulfur-containing polymer component is terminated with a silyl-functional group are disclosed in U.S. application Ser. No. 13/348,718. The silyl-functional group is removed or deblocked in the presence of moisture to expose a reactive thiol group that can be used with a variety of curing chemistries to provide a cured sealant. Other reactants and/or catalysts in these systems are unblocked. For example, in such systems, it can be desirable to use basic catalysts such as amine catalysts to accelerate the curing time of the reaction. However, during application, the unblocked basic catalyst can accelerate the reaction to cure the composition over time, thereby limiting the pot life or work life of the one-part sealant composition, where pot life refers to the time period the mixed composition remains workable for application at ambient temperature.
In sealants such as those described in U.S. Publication Nos. 2006/0270796, 2007/0287810, and 2009/0326167, a sulfur-containing polymer such as a polythioether is reacted with an epoxy curing agent in the presence of an amine catalyst to provide a cured product. Such systems typically cure in over two hours and, although exhibiting acceptable fuel resistance and thermal resistance for many applications, improved performance is desirable.
Michael addition curing chemistries are often used in acrylic-based polymer systems. Application of Michael addition curing chemistries to sulfur-containing polymers not only results in cured sealants with a faster cure rate and enhanced fuel resistance and thermal resistance, but can also provide a cured sealant with dramatically improved physical properties such as elongation. Sealant compositions employing Michael addition curing chemistries are disclosed in U.S. Application Publication No. 2013/0345371.